1. Field of the Invention
The present invention relates to an apparatus for spectroscopic analysis of a fluid medium by attenuated reflection, preferably by internal reflection spectroscopy and especially by attenuated total reflection (ATR).
The invention relates particularly to a novel apparatus for continuous reaction monitoring, for example for in-situ or on-line reaction monitoring in the chemical industry.
2. Discussion of the Background
Optical analytical methods such as transmission spectroscopy have hitherto only been used to a very limited extent for the continuous monitoring of reactions on an industrial scale. Owing to the high concentrations arising in manufacturing processes, coupled with large extinction coefficients in some cases, the pathlengths of the measuring cells would have to be on the order of 1 micrometer to obtain useful absorption spectra. It is therefore necessary to take samples and to prepare them, for example by dilution, for a measurement in the laboratory. But this process of sample preparation may alter the chemical equilibrium of the sample, so that lab results are not necessarily applicable to conditions in the reactor.
It is known that these transmission spectroscopy problems can be avoided by conducting measurements utilizing the well known optical phenomenon of the total reflection of light. When light traveling within a first medium having a refractive index n1 impinges upon a boundary between that medium and a medium of lower refractive index n2, it is totally reflected, i.e., does not pass into the second medium, when the sine of the angle of incidence xcex8 is greater than the ratio of the refractive index of the second medium to the refractive index of the first medium (sin xcex8 greater than n2/n1). Although the reflection is referred to as total, the light, owing to its wave nature, does penetrate a short distance into the second medium. The depth of penetration is usually on the order of the wavelength of the light. If the light does not interact with the second medium, then the coefficient of reflection, i.e., the ratio of the intensity of the reflected light to the intensity of the incident light, is 1 and the reflection is indeed xe2x80x9ctotalxe2x80x9d. If, however, a portion of the light which penetrates into the second medium (the so-called evanescent wave) is absorbed or scattered therein, this results in a reduced coefficient of reflection and the effect is known as xe2x80x9cattenuated total reflectionxe2x80x9d. Computing the negative decadic logarithm of the degree of transmission, i.e., the reciprocal of the coefficient of reflection R, gives the quantity of decadic extinction customary in absorption spectroscopy, which is usually known as the absorbance A:   A  =            -      log        ⁢          xe2x80x83        ⁢          (              1        R            )      
Methods and apparatuses utilizing this simple relation so as to carry out absorption measurements by attenuated total reflection in chemical analysis are known. European Patent Application EP-A-0 206 433, for example, describes an ATR probe for measuring the concentration of a light absorbing substance in a fluid medium. An optical fiber is used to couple light at a certain angle into an ATR prism where it is totally reflected one or more times at a boundary between the prism and the medium to be analyzed. The reflected light emerges from the prism via a second optical fiber which transmits the light to two detectors via a bandpass filter each. One of the filters has a transmission wavelength at which no absorption is expected in the medium, and is used as reference signal, while the other filter has a transmission wavelength at which absorption does take place in the medium. The concentration measurement is effected by comparing the measured intensity ratio with calibration measurements carried out on solutions of known concentrations.
European Patent Application EP-A-0 221 011 discloses a method for analyzing dye solutions by attenuated total reflection. This reference also describes a probelike apparatus for spectroscopic analysis of a fluid medium by attenuated total reflection. It comprises a prism which is mounted in a holder and which shares with the medium to be analyzed one or more boundaries at which incident light is totally reflected and then transmitted to a detection unit. The reference proposes various applications in the chemical industry, especially in dye manufacture.
However, existing processes and apparatuses have disadvantages. For example, absorption spectra obtainable according to the prior art depend not only on the absorption coefficient of the sample, but also on its refractive index, which may vary, for example owing to temperature changes.
It is also well known that absorption spectra obtained via ATR measurements exhibit a bathochromic shift, i.e., a shift to longer wavelengths, compared with transmission spectra. This shift is due to the fact that the refractive index n of the absorbing medium to be analyzed and hence the depth of penetration of the evanescent light is wavelength-dependent (Harrick, J. Opt. Soc. Am. 55, 851-857, 1965). Accordingly, a single determination of the coefficient of reflection of the totally reflected light is not sufficient for accurate sample analysis.
From German Patent 12 69 816 C2 a device for measuring attenuated total reflection is known comprising a goniometer which allows for changing the angle of incidence of a single light beam. The time consuming mechanical adjustment of the angle of incidence does not allow for continuously monitoring chemical reactions.
It is an object of the present invention to provide an apparatus for continuous spectroscopic analysis of fluid mediums which permits precise and economical in-line monitoring of industrial reaction processes. The apparatus of the invention shall be useful in particular in an aggressive environment at comparatively high temperatures.
We have found that this object is achieved by the apparatus of the accompanying main claim. The present invention accordingly provides an apparatus for spectroscopic analysis of a fluid medium by attenuated reflection, comprising first means for directing a first light beam onto a boundary or interface of the medium to be analyzed and means for measuring the intensity of the first light beam reflected at the boundary. The apparatus according to the invention further includes second means for directing a second light beam onto a boundary of the medium to be analyzed and means for measuring the intensity of the second reflected light beam, the first and second light beams differing in their respective polarization state and/or in their angle of incidence upon the boundary.Said first and second light beams are directed substantially simultaneously onto said boundary.
By xe2x80x9cangle of incidencexe2x80x9d is means herein, as is customary in optics, the angle between the incident light beam and the perpendicular to the boundary. By xe2x80x9clightxe2x80x9d is meant in the present context not just visible light. The apparatus of the invention is also useful in the IR region or in the UV region. The preferred wavelength region for using the method of the invention ranges from 200 nm to 20,000 nm.
The invention is predicated on the concept that the reflection of a light beam at the boundary between two dielectric media is describable by the classic Fresnel equations. It is found that the coefficient of reflection is dependent, inter alia, on the angle of incidence of the light and on its polarization state.
The apparatus of the invention makes it possible to carry out two different, mutually independent reflection measurements, so that it is possible to determine decoupled dispersion spectra n(xcex) and absorption spectra k(xcex) of the medium. These spectra, unlike those produced by conventional ATR spectroscopy, are not bathochromically shifted, since the influence of the different depths of penetration of the evanescent light on the measured reflection spectra can be corrected.
Unlike the apparatuses known from EP-A-0 206 433 and EP-A-0 221 011, the apparatus of the invention is not restricted to angles of incidence which are larger than the limiting angle of total reflection. This is because Fresnel""s equations show that, for smaller angles too, the coefficient of reflection is dependent not only on the refractive index n(xcex) but also on the absorption coefficient k(xcex) of the medium to be analyzed, i.e., when absorption occurs in the medium it is not just the beam passing through the medium which is attenuated but also the beam reflected at the boundary.
In a first embodiment of the apparatus of the invention, the angle of incidence of the first light beam on the boundary is xcex81 and the angle of incidence of the second light beam on the boundary is xcex82, which differs from xcex81. Both the beams are reflected at the boundary and conducted to suitable detection means.
This apparatus makes it possible, for example, to measure the total intensity in the case of beams reflected at these two angles. The wavelength-dependent coefficients of reflection for the two angles can be used, as will be shown hereinafter, to assign numerical values to n(xcex) and k(xcex). There is no need for polarizers, so that the apparatus is relatively inexpensive to manufacture and can be used especially under harsh conditions in chemical reaction monitoring, for example at high temperatures and pressures and in an aggressive environment. Since it is only total intensities which are still measured in a polarization-independent way, there is no need either for costly polarization-preserving optical fibers. The apparatus of the invention provides an exact determination of the refractive index n and of the absorption coefficient k, which depends only on the accuracies of the reflection measurements and of the calculation and therefore permits in particular the exact description of an absorption spectrum or a dispersion curve of the medium to be analyzed. Simple optical components can be used, especially relatively inexpensive multimode optical fibers.
xe2x80x9cTotal intensityxe2x80x9d for the purposes of the present invention is to be understood as meaning a measurement of the polarization-independent intensity of the reflected light.
The angles of incidence of the two light beams preferably differ by an amount in the range from 5 to 20xc2x0, particularly preferably by about 10xc2x0.
In one variant, the angle xcex81 is larger than the limiting angle of total reflection while the angle xcex82 is smaller than the limiting angle of total reflection. In this case, the intensity of the first reflected beam is more dependent on k(xcex), while the intensity of the second reflected beam is more dependent on n(xcex).
In a particularly preferred variant, however, both the angles xcex81 and xcex82 are larger than the limiting angle of total reflection. This represents the case of an ATR probe, but the ATR probe of the invention differs from prior art probes in that it utilizes two measuring beams.
The apparatus of the invention is preferably constructed as an elongate probe comprising a cylindrical protective housing at whose free end face is disposed a prism having at least one face which is wettable by the medium to be analyzed and which forms the boundary for the reflection of the two light beams. The prism/medium boundary is a clearly defined, planar surface, as is preferred for the impingement of light on an air/medium boundary.
However, there may also be a separate boundary with the medium for each light beam. These two boundaries may be realized by one prism or else by two prisms.
Advantageously the prism is mounted replaceably in the probe. For each measuring problem it is then possible to select an appropriate prism. Different prism geometries make it possible, for example, to use different angles of incidence. Different prism materials permit a selection with regard to transmission properties and refractive index.
The cylindrical protective housing is advantageously formed of chemically resistant metallic or ceramic materials. In this way it is possible to realize dip probes up to 2.5 m in length for use on an industrial production scale. In a particularly preferred embodiment, the prism is forced by suitable elastic means, for example compression springs, against sealing means surrounding an opening in the end face of the cylindrical housing. The compression springs serve to compensate the longitudinal expansion of the probe in the event of an increase in the temperature and maintain a good seal with regard to the prism, since the necessary contact pressure is ensured even at high temperatures. The probe of the invention can therefore also be used in the case of process temperatures of 200xc2x0 C. or more or in the case of wildly fluctuating temperatures. An example of a useful sealing means is a gasket in the shape of a circular ring. This design makes it possible to fabricate dip probes more than two meters in length with just one seal. Such comparatively long probes are preferably installed in customary dip tubes, i.e., gas inlet tubes, so as to obtain good mechanical stability in stirred vessels, for example.
The prism is preferably a crystal which, for measurements in the UV to near IR region, is composed of materials which have a high refractive index and are substantially chemically resistant, such as quartz glass, sapphire or diamond, zirconium oxide or zirconia (doped zirconium oxide). For infrared measurements it is preferable to employ semiconductor crystals, for example ZnSe crystals. Preferably the entire optical device is fixedly mounted, eliminating the need for reconfiguration and adjustment.
For an ATR prism composed of quartz and refractive indices in the range from 1.2 to 1.7 for the mediums to be analyzed, the angles of incidence are preferably in the range from 55 to 60xc2x0 for the first beam and in the range from 65 to 70xc2x0 for the second beam. Preferably the prism has an essentially frustoconical shape and comprises a light entry and exit face parallel to the boundary, a first pair of mirror-symmetrically opposite side faces which form an angle of xcex81/2 with the normal to the boundary, and a second pair of mutually opposite mirror-symmetrical side faces which form an angle of xcex82/2 with the normal to the boundary.
In this embodiment of the apparatus according to the invention, the two light beams pass essentially vertically through the horizontal light entry and exit face into the prism, are reflected at one of the side faces of the first or second side-face pair and impinge at an angle of xcex81 or xcex82, respectively, upon the boundary, where they are diverted to the other side face of each pair. There, they are vertically reflected upward and emerge from the prism with a parallel offset relative to the incident beam. To avoid light loss, the reflection at the side faces is total as well. In one variant, the prism is arranged in such a way that one or both of the side faces of a pair are likewise wetted by the medium to be analyzed, so that each light beam undergoes two or three xe2x80x9cattenuatedxe2x80x9d reflections. In a particularly preferred variant, however, only the lower boundary is wetted by the medium. In this case, the prism holder will be constructed in such a way that no absorption of the evanescent light takes place at the side faces in order that the measurement of the medium may not be distorted.
Advantageously the first side-face pair is twisted by 90xc2x0 around a normal to the boundary relative to the second side-face pair, resulting in a particularly compact apparatus.
In a second embodiment of the apparatus according to the invention, the optical path of the first light beam includes pre-boundary a polarizer for perpendicularly polarized light and/or post-boundary an analyzer for perpendicularly polarized light and the optical path of the second light beam includes pre-boundary a polarizer for parallel-polarized light and/or post-boundary an analyzer for parallel-polarized light. As light is reflected at the boundary, a certain amount of depolarization may occur. It is therefore preferable in this case to use both a polarizer and an analyzer for each light beam.
What is measured in this embodiment is not, as in the prior art, the total intensity of the reflected light, but, separately, the intensities of the two polarization directions of the light parallel to the plane of incidence Ip and perpendicular to the plane of incidence Is. For this case, Fresnel""s equations have been analytically solved for the desired parameters of the medium, i.e., for the refractive index n and the absorption coefficient k (Querry, xe2x80x9cDirect Solution of Generalized Fresnel Reflectance Equationsxe2x80x9d, J. Opt. Soc. America, 59 (1969), 876-877). The angles of incidence of the two beams may be identical. In principle it would also be possible here to use just one light beam and to switch the polarizers and analyzers or, for example in the case of the use of polarizing films, turn them by 90xc2x0 between two successive measurements.
This embodiment of the probe according to the invention is advantageous in that it makes use of the known analytical solutions to enable the desired spectra to be calculated directly and rapidly from the measurements. However, the use of polarizers or analyzers is mandatory.
In a third embodiment of the apparatus according to the invention, the optical path of the first and second light beams include pre-boundary polarizers for perpendicularly polarized light and/or post-boundary analyzers for perpendicularly polarized light. The light beams impinge upon the boundary at different angles of incidence. Again, it is preferable to use both polarizers and analyzers.
For this case of a reflection measurement with perpendicularly polarized light and two different angles of incidence, an iteration method has been described for solving the Fresnel equations (Fahrenfort and Visser in Spectrochim. Acta 18, 1103-1116 (1962)).
In the probe of the invention, the light is advantageously not focused on the boundary; instead the light should impinge upon the boundary in as parallel a state as possible, for exact compliance with the chosen angle of incidence. It is therefore particularly advantageous to provide first optical fibers for passing the incident light beams onto the boundary and second optical fibers for directing the light beams reflected at the boundary to the means for measuring the light intensities, in which case collimating means are disposed between the light exit faces of the first optical fibers and the boundary, and between the boundary and the light entry faces of the second optical fibers, respectively for coupling the light beams in and out, respectively.
In an embodiment of the apparatus according to the invention, which for cost reasons is particularly preferred for use as a monitoring probe in chemical production, the measurement is carried out polarization-independently at two different angles. This is because high quality polarizers capable of withstanding temperatures of above 200xc2x0 C. are very costly and therefore uneconomical for many monitoring duties. Also, the use of reflection probes in process monitoring has gained popularity in recent years especially because the use of optical fibers made it possible to have the probe head with the prism and the actual measuring unit (spectrometer, sensor and microprocessor) far apart. Polarization-dependent measurements would therefore also require the use of costly polarization-preserving single mode fibers.
A measuring process using a probe according to the invention will now be more particularly described by way of example. A polarization-independent measurement is carried out at two different angles of incidence. Total reflection shall occur at both angles of incidence, i.e., an ATR probe is used.
A first light beam is in a conventional manner allowed to impinge under total reflection upon the boundary between the prism and the medium to be analyzed, at a first angle of incidence xcex81. The total intensity I1 of the totally reflected light beam is measured at a certain wavelength l. According to the invention, in addition, a second light beam is allowed to impinge under total reflection upon the boundary at an angle of incidence xcex82, which differs from the first angle of incidence xcex81, and the total intensity I2 of the totally reflected second light beam is measured. The two measurements are then used to calculated the absorption coefficient k(xcex) and/or the refractive index n(xcex).
When used for monitoring industrial chemical processes, each beam is usually totally reflected at the boundary only once. In thin, weakly absorbing mediums, however, multiple total reflection at the boundary may also be provided for in one variant. A measuring arrangement suitable for this purposexe2x80x94albeit for measurements at only one angle of incidencexe2x80x94is described in EP-A-0 206 433, for example.
The accuracy of wavelength measurement depends on the particular purpose. For spectroscopic studies, for example, the need for a higher resolution will require the use of a grating spectrograph positioned in front of a diode array or other polarization-independent detection means. If, for example, there is only interest in monitoring the formation of a certain reaction component, then there is no need to record a spectrum and instead, for example, suitable bandpass filters can be used to isolate a characteristic wavelength range in which the absorption characteristics of the properties are expected to change as the reaction proceeds.
The two light beams are preferably measured simultaneously or substantially simultaneously, for example by providing a separate detection array for each reflected light beam.
Such a setup can also be used to simultaneously measure a plurality of wavelengths, i.e., to record a larger region of the spectrum. Useful sensitive diode arrays are known and typically have 256, 512 or 1024 diodes (the MMS or MCS arrays from Zeiss, for example). The signals from the diodes are amplified and processed by a microprocessor. The choice of diode arrays can be influenced by numerous factors, for example the desired resolution, the time available for on-line evaluation, the computing power, the accuracy required for measurement and computation, etc.
To calculate the constants n and k of interest, the present example utilizes polarization-independent detectors to measure the total intensity of the reflected light. The coefficient of reflection for a given angle of incidence is then made up as per             R      θ        ⁢          (              n        ,        k            )        =                    R        s            2        ⁢          xe2x80x83        ⁢          (              1        +                              R            p                                R            s                              )      
from the perpendicularly polarized fraction Rs and the parallel-polarized fraction Rp. According to Fresnel""s formulae, the coefficient of reflection is also dependent on the angle of incidence xcex8, so that different coefficients of reflection Rxcex81 and Rxcex82 result for the two light beams. An analytical solution of Fresnel""s formulae for n and k is not possible for this case. The actual values of n and k have to satisfy the following two relations simultaneously:
Rxcex81(n,k)=R1
and
Rxcex82(n,k)=R2
i.e., the theoretical coefficient of reflection Rxcex81 has to be equal to the measured coefficient of reflection R1 of the first light beam for n and k. A corresponding relation has to be satisfied for the second light beam for the same n and k.
To solve this nonlinear system of equations, the invention proposes forming a function F(n,k) of the following structure:
F(n,k)=(Rxcex81(n,k)xe2x88x92R1)2+(Rxcex82(n,k)xe2x88x92R2)2
i.e., the sum of the advantageously squared differences of the two theoretical coefficients of reflection Rxcex8 and of the respective measured coefficient of reflection R, and numerically minimizing F. The squaring of the difference terms leads to a continuous function F, facilitating minimization. Rxcex8 is obtained from Fresnel""s formulae by the following relation:                     R        θ            ⁢              (                  n          ,          k                )              =                  1        2            ⁢              xe2x80x83            ⁢                                                  (                              a                -                                  cos                  ⁢                                      xe2x80x83                                    ⁢                  θ                                            )                        2                    +                      b            2                                                              (                              a                +                                  cos                  ⁢                                      xe2x80x83                                    ⁢                  θ                                            )                        2                    +                      b            2                              ⁢              xe2x80x83            ⁢              (                  1          +                                                                      (                                      a                    -                                          sin                      ⁢                                              xe2x80x83                                            ⁢                      θ                      ⁢                                              xe2x80x83                                            ⁢                      tan                      ⁢                                              xe2x80x83                                            ⁢                      θ                                                        )                                2                            +                              b                2                                                                                      (                                      a                    +                                          sin                      ⁢                                              xe2x80x83                                            ⁢                      θ                      ⁢                                              xe2x80x83                                            ⁢                      tan                      ⁢                                              xe2x80x83                                            ⁢                      θ                                                        )                                2                            +                              b                2                                                    )                                where        ⁢                  xe2x80x83                ⁢        a            -              ⅈ        ⁢                  xe2x80x83                ⁢        b              =                            m          2                -                              sin            2                    ⁢          θ                          m    =                  (                  n          -                      ⅈ            ⁢                          xe2x80x83                        ⁢            k                          )                    n        0            
where m is the complex refractive index of the medium and n0 is the refractive index of the optical element, i.e., of the prism, for example. Algorithms for minimizing a nonlinear function having two parameters are known. Particular preference is given to using a Broyden-Flatcher-Goldfarb-Shanno minimization algorithm (unconstrained quasi-Newton minimization) as described for example in the standard work xe2x80x9cNumerical Recipes in Cxe2x80x9d and implemented in the MATLAB Optimization Toolbox, The Math Works Inc.
The MATLAB Toolbox optimization technique requires a computing time of about 19 seconds on a 133 MHz Pentium computer (PENTIUM(copyright) is a registered trademark of Intel) for 256 spectral points and an estimated error of 10xe2x88x924 in the determination of n and k.
The evaluation time can be reduced, depending on requirements. For example, it is possible to use faster processors or to use a plurality of processors in a parallel array.
Advantageously the coefficients of reflection R1 and R2 following r1 and r2 total reflections (usually: r1=r2=1) are determined from the measured intensities I1 and I2 according to the following relation:             R              1        ,        2              ⁢          (      λ      )        =            (                                    I                          1              ,              2                                ⁢                      (            λ            )                                    I          ref                    )              1              r1        ,        2            
The reference intensity Iref corresponds essentially to the intensity I0 of the light source used. Since a light source may be subject to intensity fluctuations in the course of a measurement, it is advantageous to measure the reference intensity continuously as well. This may be accomplished, for example, by incorporating in the spectrometer a third diode array for wavelength-dependent measurement of the intensity I0. For particularly accurate measurements, the intensity I0 will be additionally weighted using the transmission curve of the entire optical system, advantageously by also taking account of the grating function of the spectrometer. This is because the transmission curve of the measuring system will generally also show a certain degree of wavelength dependence, which will be particularly noticeable when the region of the spectrum measured is relatively large. The corresponding transmission curve Itrans(xcex) is determined once for a given setup and can then be stored for the evaluation of later measurements.
The apparatus of the invention is particularly useful for applications in the chemical industry. Typical examples are continuous concentration measurements or absorption-spectroscopic studies of chromophoric systems in dye synthesis, in the manufacture of paints and generally in the processing of highly concentrated organic substances. The method of the invention provides for the first time the possibility of precise in-situ concentration measurements of substances which absorb strongly in the UV region, such as hydrosulfite, benzaldehyde or styrene, by preparing linear calibration curves.
The reflection measurements which can be performed using the probe of the invention are insensitive to solid particles more than a few micrometers in diameter, which is particularly advantageous in the case of measurements where there is a danger that comparatively large particles will distort the measurement. On the other hand, fine pigment dispersions in paints or printing inks can be reliably investigated.